In thrusters of this kind, usually in the form of a propeller device arranged in a tunnel in the vessel's hull, the propeller device or devices are, in accordance with the current state of the art, mounted when the vessel is built. In these known propeller devices, which are based on oil-lubricated bearings and gearwheels, it is not possible to remove the propeller device for service, repair or to replace the propeller device, should this be necessary, without going into dock. In such cases, major work must usually be carried out on the vessel's hull in order to be able to remove the propeller device, and this is time-consuming and costly.